Method of forming a stack of layers using a sacrificial layer

ABSTRACT

The disclosed technology generally relates to forming a semiconductor structure and more particularly to forming a stack of layers of a semiconductor structure using a sacrificial layer that is removed during deposition of a functional layer. In one aspect, the disclosed technology relates to a method of protecting a top surface of a layer in a semiconductor structure. The method comprises: providing the layer on a substrate, the layer having an initial thickness and an initial composition; forming a sacrificial metal layer on and in contact with the layer, the sacrificial metal layer comprising a light metal element; and depositing by physical vapor deposition a functional metal layer on and in contact with the sacrificial metal layer. The sacrificial metal layer is removed by sputtering during the deposition of the functional metal layer, such that an interface is formed between the layer and the functional metal layer. The sacrificial metal layer protects the layer during the deposition of the functional metal layer, such that the layer has a final thickness which substantially matches the initial thickness and a final composition which substantially matches the initial composition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims foreign priority to European Patent ApplicationNo. 15160520.1, filed Mar. 24, 2015, the content of which isincorporated by reference herein in its entirety.

BACKGROUND

Field

The disclosed technology generally relates to forming a semiconductorstructure and more particularly to forming a stack of layers of asemiconductor structure using a sacrificial layer.

Description of the Related Technology

Semiconductor technology continues to integrate increasingly smallerfeature sizes, and correspondingly continues to increase the devicedensity and functionality per unit area. Increasing device density inturn enables proportional reductions in device footprint and cost. Thetrend of downscaling, however, continues to introduce new and previouslyunmet challenges with each successive generation.

Generally, semiconductor device fabrication involves formation ofvarious semiconductor structures by patterning, e.g., by forming (e.g.,depositing) and removing (e.g., etching) one or more layers ofmaterials. When forming a semiconductor structure having a plurality oflayers of materials, each layer may be selected to provide a particularfunctional feature within the semiconductor structure. Individual layersare generally deposited during fabrication of the semiconductorstructure to form various functional structures. For example, a metallayer may be deposited to form, e.g., metallization and electricalcontacts, among other electrically conducting structures, while adielectric layer may be deposited to form, e.g., isolation structuresand spacer structures, among other electrically insulating structures.

Generally, smaller devices are more prone to being detrimentallyaffected by defects, which may lead to, e.g., increased current leakageand charge trapping. Even relatively small variations in thenanometer-scale of the material structure/composition and the interfacesbetween the layers may substantially degrade the device performance. Asa result, the smaller feature sizes increase the need for advancedfabrication techniques with reduced defects.

Physical vapor deposition (PVD) is a fabrication technique used todeposit thin layers in most semiconductor devices by condensation of avaporized target material onto a substrate. To meet the increasing needsof physical scaling of semiconductor devices, there is a correspondingneed to improve PVD techniques for fabrication of various structures bydepositing thin films with improved purity, improved thickness controland smoother interfaces between adjacent layers.

There is therefore a need for improved techniques for formingsemiconductor structures comprising a plurality of layers using PVD.

SUMMARY OF CERTAIN INVENTIVE ASPECTS

It is an object of the disclosed technology to provide an improvement ofthe above techniques and prior art.

In one aspect, a method comprises: providing a layer on a substrate, thelayer having an initial thickness and an initial composition, and a topsurface; depositing by physical vapor deposition (PVD) a sacrificialmetal layer on and in contact with the top surface, the sacrificialmetal layer comprising a light metal element; and depositing by physicalvapor deposition a functional metal layer on and in contact with thesacrificial metal layer. The sacrificial metal layer is substantiallyremoved by sputtering during the deposition of the functional metallayer such that an interface is formed between the layer, e.g., the topsurface of the layer, and the functional metal layer. As used herein, asacrificial metal layer that is substantially removed may have a finalthickness that is about +/−5% of an initial thickness. The sacrificialmetal layer substantially protects the top surface during the depositionof the functional metal layer such that the layer has a final thicknesssubstantially matching the initial thickness and a final compositionsubstantially matching the initial composition. As used herein, a finalthickness that substantially matches an initial thickness may be withinabout +/−5% of the initial thickness. As used herein, a finalcomposition that substantially matches an initial composition may bewithin about 95% of the initial composition with respect to each of theconstituent element.

The present disclosure is based on the realization that the use of asacrificial metal layer may reduce damages to a layer when depositing afunctional metal layer on top of the layer.

Initially, a layer is provided on a substrate. The layer may be provideddirectly or indirectly on the substrate. In other words, other layers orstructures may be provided between the substrate and the layer. Thelayer provided on the substrate has an initial thickness and an initialcomposition. Further, the layer has a top surface. The top surface isthe surface of the layer facing away from the substrate.

A sacrificial metal layer is deposited on and in contact with the topsurface by physical vapor deposition (PVD). In other words, asacrificial metal layer is deposited in physical contact with the topsurface. The sacrificial metal layer comprises a light metal element.

It should be noted that within the context of this application the term“light metal element” may be any metal element having an atomic numberof 36 or less. Hence, a light metal element may be any metal element inthe first four periods of the periodic table.

A functional metal layer is deposited on and in contact with thesacrificial metal layer by PVD. During the deposition of the functionalmetal layer, the sacrificial metal layer is removed by sputtering,resulting in that an interface is formed between the top surface and thefunctional metal layer. The sputtering is caused by the physical vapordeposition of the functional metal layer, when the atoms forming thefunctional metal layer impinges on the sacrificial metal layer. Thesputtering of the sacrificial metal layer involves two major mechanismsremoving the atoms of the sacrificial metal layer. The first mechanismis physical sputtering of the sacrificial metal layer and the secondmechanism is evaporation of the sacrificial metal layer. In order toachieve the sputtering of the sacrificial metal layer the atoms formingthe functional metal layer need to transfer a sufficient impulse to theatoms of the sacrificial metal layer. The impulse transferred is mainlydetermined by the velocity and weight of the atoms forming thefunctional metal layer.

The sacrificial metal layer protects the top surface during thedeposition of the functional metal layer. Hence, the atoms forming thefunctional metal layer will impinge on and sputter the sacrificial metallayer, such that top surface is not affected or only affected to alimited amount by the atoms forming the functional metal layer. Theprotective effect caused by the sacrificial metal layer results in that,the layer having the top surface has a final thickness matching orsubstantially matching the initial thickness and a final compositioncorresponding to the initial composition.

As used to describe various features herein, the terms “final thickness”and “final composition” refer to the thickness and the composition aftercompletion of the deposition of the functional metal layer. As usedherein, a thickness substantially matching another thickness, such as afinal thickness substantially matching an initial thickness may refer toa situation where the final thickness corresponds to about ±5% of theinitial thickness. In addition, the “final composition corresponding tothe initial composition” may refer to a final composition where at leastto 95% of the composition with respect to each element corresponds tothose of the initial composition.

Besides, the protective effect results in that, the intermixing of thetop surface and the functional material during deposition of thefunctional layer is reduced.

According to embodiments, the layer may comprise Co, Fe, B, or acombination thereof, which is advantageous in that the method may beused with magnetic layers.

According to embodiments, the sacrificial metal layer may comprise Mg,Al, Ca, Zn, or a combination thereof, which is advantageous in that thesacrificial metal layer may be removed by sputtering during thedeposition of the functional metal layer. Moreover, by this arrangementthe risk of damaging the top surface of the layer may be minimized asMg, Al, Ca and Zn are light metal elements which may be deposited on thetop surface with a reduced risk of damaging the top surface. Further,Mg, Al, Ca and Zn efficiently protect the top surface during thedeposition of the functional metal layer. Preferably Mg is used for thesacrificial metal layer.

According to embodiments, the functional metal layer may be an amorphousmetal layer.

According to embodiments, the functional metal layer may comprise Ta.

According to embodiments, the thickness of the sacrificial metal layermay be smaller than three times the thickness of the functional metallayer. By this arrangement, the sacrificial layer may be substantiallycompletely removed during the deposition of the functional metal layer.For instance, when using Ta in the functional metal layer and Mg in thesacrificial metal layer, 2-3 Å of Mg is removed during the deposition of1 Å Ta. In other words, the sputter yield has a factor of 2-3. Aspecific factor will be determined by the respective elements and systemparameters used.

According to embodiments, the thickness of the sacrificial metal layermay be smaller than 2 nm, which is advantageous in that the sacrificiallayer may be completely removed during the deposition of the functionalmetal layer.

According to embodiments, the method may further comprise annealing thesemiconductor structure at an elevated temperature of 250° C.-400° C.,wherein the layer at least partially crystallizes into a [001]orientated layer, which is advantageous in that desired magneticproperties of the layer may be achieved. A [001] orientated layer mayresult in an increased tunnelling magneto resistance, TMR.

According to embodiments, the layer may have perpendicular magneticanisotropy, which is advantageous in that the layer may exhibit desiredmagnetic properties.

According to embodiments, the layer and the functional metal layer mayform part of a spin-transfer torque magnetoresistive random-accessmemory stack, STT-MRAM stack. By incorporating the layer and thefunctional metal layer in a STT-MRAM stack, an improved memory may beachieved.

According to embodiments, the method may further comprise forming amagnetic tunnel junction (MTJ) by means of physical vapor deposition,wherein the layer forms part of the magnetic tunnel junction. By thisarrangement, a memory device having improved read, write and stabilityproperties may be achieved.

According to embodiments, the method may further comprise forming anelectrode adjacent to the functional metal layer, which is advantageousin that the electrode may be used to contact the functional metal layerand hence a device in which the functional metal layer may beincorporated.

According to embodiments, the layer may comprise a high-k dielectricmaterial or an oxide. By this arrangement, the method may advantageouslybe used to fabricate various types of devices. More specifically, themethod may advantageously be used when the formation of an interfacebetween a high-k dielectric or an oxide material and another material iscrucial in the sense that the high-k dielectric or the oxide materialneed to be protected during the formation of other material such thatthe risk of damaging the high-k dielectric or the oxide material isreduced.

The functional metal layer may comprise Co, Ti, W, Mo, Ru, Hf, or acombination thereof which is advantageous in that the method may be usedto fabricate various types of devices comprising Co, Ti, W, Mo, Ru orHf.

Further features of, and advantages with, the present disclosure willbecome apparent when studying the appended claims and the followingdescription. The skilled person will realize that different features ofthe present disclosure may be combined to create embodiments other thanthose described in the following, without departing from the scope ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other aspects of the present disclosure will now be describedin more detail, with reference to the enclosed drawings showingembodiments of the invention.

FIG. 1a illustrates an intermediate semiconductor structure prior toforming a functional layer, according to embodiments of the presentdisclosure.

FIG. 1b illustrates an intermediate semiconductor structure prior toforming a functional layer, according to embodiments of the presentdisclosure.

FIGS. 2a and 2b illustrates a semiconductor structure as part of oneimplementation of spin-torque transfer magnetic random access memories(STT-MRAMs), according to embodiments.

FIGS. 3a and 3b illustrate measured tunnelling magnetoresistance ratios(TMR) of STT-MRAM structures fabricated using a method according toembodiments, plotted as function of resistance area product, RA, andthickness.

FIGS. 4a, 4b and 4c illustrate measurements for STT-MRAM structuresfabricated using a method according to embodiments, illustratingsputtering of Mg during Ta deposition.

FIG. 5 illustrates measurements of magnetoresistance as function ofannealing temperature obtained on CoFeB layers onto which Ta layers havebeen deposited, according to embodiments.

FIG. 6 illustrates a method of protecting a top surface of a layer in asemiconductor structure, according to embodiments.

DETAILED DESCRIPTION OF CERTAIN ILLUSTRATIVE EMBODIMENTS

The present disclosure will now be described more fully hereinafter withreference to the accompanying drawings, in which currently preferredembodiments of the disclosure are shown. This disclosure may, however,be embodied in many different forms and should not be construed aslimited to the embodiments set forth herein. These embodiments arerather provided for thoroughness and completeness, and for fullyconveying the scope of the disclosure to the skilled person.

In the following a semiconductor structure and a method for protecting atop surface of a layer in the semiconductor structure will be described,with reference to FIGS. 1a and 1 b.

FIG. 1a illustrates a pre-stage of a semiconductor structure 100,according to embodiments. The pre-stage of the semiconductor structure100 comprises a substrate 102, a layer 104 and a sacrificial metal layer106. The layer 104 has an initial thickness, t₀, and an initialcomposition C₀, and a top surface 108.

The sacrificial metal layer 106 can be provided by a depositiontechnique, e.g., physical vapor deposition (PVD) whereby the sacrificialmetal layer 106 is formed on and in contact with the top surface 108 ofthe layer 104. The top surface 108 is the surface of the layer facingaway from the substrate. However, other embodiments are possible, wherethe sacrificial metal layer is formed by other techniques, e.g.,chemical vapor deposition, evaporation or electroplating, among otherpossible techniques for forming a metal layer.

The sacrificial metal layer 108 comprises, as will be further discussedbelow, a light metal element.

FIG. 1b illustrates a semiconductor structure 200 resulting afterfurther processing of the pre-stage of the semiconductor structure 100.The semiconductor structure 200 comprises the substrate 102, the layer104 and a functional metal layer 110.

To form the semiconductor structure 200 from the pre-stage of thesemiconductor 100, the functional metal layer 110 is provided by aphysical deposition technique, such as physical vapor deposition (PVD).As used herein, a physical deposition technique refers to a technique inwhich the depositing atoms have more than thermal energy (i.e., kT).While PVD is an example of a physical deposition technique, certainother deposition techniques such as certain chemical vapor deposition(CVD) techniques can be also be used to practice the methods describedherein. For example, when the substrate is electrically biased,depositing atoms in a CVD process may have more than thermal energy.

The functional metal layer 110 is thereby deposited on and in contactwith the sacrificial metal layer 106. During the depositing of thefunctional metal layer 110 the sacrificial metal layer 106 is at leastpartially removed by sputtering such that an interface 112 is formedbetween the top surface 108 and the functional metal layer 110. While itis removed during the deposition of the functional metal layer 110, thesacrificial metal layer 106 protects the top surface 108 during thedeposition of the functional metal layer 110. For example, thesacrificial layer 106 may be gradually removed during the deposition ofthe functional metal layer 110 such that the top surface of theunderlying layer 104 remains substantially covered by the sacrificialmetal layer 106 until towards the end of the duration of the depositionof the functional metal layer 110. In other words, the sacrificial metallayer 106 thereby acts as a protecting layer, or as a cushion, reducingphysical effects on the top surface 108 during the sputtering of thefunctional metal layer 110.

In some embodiments, an improved quality of the layer 104 is therebyachieved by substantially removing the sacrificial metal layer 106 byion bombardment during the deposition of the functional metal layer 110.In other embodiments, an improved quality of the layer 104 is achievedby entirely removing the sacrificial layer 106 by ion bombardment. Itwill be appreciated that, taking into account of of certain statisticalor other physical variations during the course of semiconductorprocessing, the sacrificial layer 106 may be described to besubstantially removed when, on average across the sacrificial layer 106,a final thickness is about +/−5% of an initial thickness, for instanceabout 0%. It will be also be appreciated that when the sacrificial layer106 is described to be substantially removed, it may be completelyremoved from some portions of the layer while remaining on otherportions, such that on average, the final thickness is about +/−5% ofthe initial thickness, for instance about 0%.

In some embodiments, an improved quality of the layer 104 is achieved bysubstantially preventing the underlying layer 104 from being removed byion bombardment during the deposition of the functional metal layer 110.In other embodiments, an improved quality of the layer 104 is achievedby entirely preventing the underlying layer 104 from being removed byion bombardment. It will be appreciated that, taking into account of ofcertain statistical or other physical variations during the course ofsemiconductor processing, the layer 104 may be described to besubstantially prevented from being removed when, on average across thelayer 104, a final thickness is within about +/−5% of an initialthickness, for instance about 0% of the initial thickness. It will bealso be appreciated that when the layer 104 is described to besubstantially removed, it may be completely prevented from being removedfrom some portions of the layer while incompletely prevented from beingremoved in other portions, such that on average the a final thickness iswithin about +/−5% of the initial thickness, for instance within about0% of the initial thickness. In this respect, in some embodiments, thelayer 104 has a final thickness, t_(f), which substantially matches aninitial thickness, t₀.

In some embodiments, when the underlying layer 104 is substantiallyprevented from being removed by ion bombardment, its composition, e.g.,its surface composition, remains substantially unchanged. In otherembodiments, its composition, e.g., its surface composition, remainsentirely unchanged. It will be appreciated that, because of certainstatistical or other physical variations, the composition of the layer104 may be described to be substantially unchanged when, on averageacross the layer 104, a final concentration of each chemical element iswithin about +/−5% of an initial concentration. In this respect, in someembodiments, the layer 104 has a final composition, C_(f), whichsubstantially matches an initial composition C₀.

Atoms forming the functional metal layer 110 may, during the PVDprocess, impinge on and thereby sputter the sacrificial metal layer 106,while substantially not impinging the layer 104 such that the topsurface 108 of the layer 104 is not affected or only affected to alimited amount by the atoms forming the functional metal layer 110. Thisis a result of the sacrificial metal layer 106 protecting the topsurface 108 during the deposition of the functional metal layer 110. Thelayer 104 having the top surface 108 has therefore a final thickness,t_(f), matching or substantially matching the initial thickness, t₀, anda final composition, C_(f), matching or substantially matching to theinitial composition, C₀. An improved interface 112 may thereby be formedbetween the top surface 108 and the functional metal layer 110.

The structure of FIGS. 1a and 1b may, respectively, form parts of apre-stage of a semiconductor structure device or a semiconductorstructure device. FIG. 2a illustrates a semiconductor structure deviceconfigured as part of a spin-torque transfer magnetic random accessmemory (STT-MRAM), according to some embodiments.

STT-MRAMs, like the STT-MRAM 300, are attracting much attention becauseof their potential in offering a reduction of power consumption throughnon-volatility and a reduction of the size of memory cells compared toconventional memory cells.

In various embodiments, the STT-MRAM device 300 comprises a magnetictunnel junction, MTJ, 302 providing a tunnel magneto resistance, TMR. Itis further known that some MTJs may have perpendicular magneticanisotropy (PMA) such that a so called p-MTJ is formed. A p-MTJ has amagnetic anisotropy that is perpendicularly oriented relative to thedirection of tunnelling of spin polarized electrons, or relative to amain surface of the free layer and the fixed layer of the MTJ. The useof a p-MTJ generally offers an improved current switching and highthermal stability for the STT-MRAM device 300.

The MTJ 302 may be formed by means of physical vapor deposition. The MTJ302 comprises a stack of layers 304, 306, and 308. The stack of layersmay, for example, comprise CoFeB, MgO, and CoFeB, respectively. TheCoFeB layers 304 and 308 form the magnetic layers and the MgO layer 306forms the insulating layer, also referred to as the tunnelling layer, ofthe MTJ 302.

Again referring to FIGS. 1a and 1b , the CoFeB layer 304 is to beunderstood to correspond to the layer 104.

The CoFeB layers 304 and 308 have preferentially [001] textures in orderto obtain a high TMR. To this end, a Co/Ni perpendicular syntheticanti-ferromagnet (p-SAF) structure 310 is provided, which is arranged topin the magnetization the CoFeB layer 308. The p-SAF structure 310 mayalso be referred to as a reference layer (RL).

The CoFeB layer 304 is the switching element of the STT-MRAM device 300and is commonly referred to as a free layer (FL).

In various embodiments, the STT-MRAM device 300 further comprises afunctional metal layer 312 arranged on top of and in contact with theCoFeB layer 304. The functional metal layer 312 is an amorphous metallayer. The functional metal layer 312 comprises Ta or is formed of Ta.It should, however, be noted that the functional metal layer may inother embodiments comprise an alloy comprising Ta or be formed by Ta.The metal layer may moreover be a crystalline metal layer.

The STT-MRAM device 300 may further comprise an electrode 314 formedadjacent to the functional metal layer 312.

The electrode 314, which may be referred to as a top electrode, maycomprise Ru/Ta/Ru.

The skilled person in the art realizes that Ta may be used at variousother positions within the STT-MRAM 300. An additional Ta layer 316 maytherefore for instance be used as an amorphous interlayer to avoidtexture transfer from the (111) textured FCC Co/Pt or Co/Ni based p-SAFstructure 310 to the (001) textured BCC CoFeB layer 308 during acrystallization anneal.

Besides, an additional Ta layer might be used as part of the free layerto form a CoFeB/Ta/CoFeB free layer structure.

To this end, a semiconductor structure such as a STT-MRAM structure, maycomprise an oxide layer. A MgO layer may for instance be used as acapping layer arranged on top of a CoFeB layer as the formed MgO/CoFeBinterface may enhance the perpendicular anisotropy of the STT-MRAMstructure. The skilled person realizes that the use of a sacrificiallayer comprising for instance Mg may be used to efficiently protect theoxide layer during deposition of a functional metal layer on top of theoxide layer.

To provide efficient charge transfer in the STT-MRAM 300 a bottomelectrical contact 318 is further arranged below the p-SAF structure310.

The skilled person in the art realizes that according to otherembodiments a Co/Pt, or Co/Pd p-SAF structure may be used. FIG. 2billustrates a STT-MRAM 400 comprising a Co/Pt p-SAF structure 310. Thestructure of the STT-MRAM 400 is similar to the structure of theSTT-MRAM 300, why its construction and function will not be described indetail in order to avoid undue repetition.

It should be noted that the properties and performance of the STT-MRAMs300, 400 depends critically of the quality of their MTJs 302, which inturn are sensitive to the quality of the functional metal layer 312.

The depicted p-MTJ's 302, of FIGS. 2a and 2b , on the bottom pinnedp-SAF's 310 were deposited in-situ by physical vapor deposition, PVD, ina 300 mm cluster tool, EC7800 Canon-Anelva, on TaN-based bottomelectrodes 318 on a substrate (not shown). The bottom electrodes 318were moreover smoothened down to 0.05 nm root mean square roughness viachemical mechanical polishing, CMP prior to depositing the respectivep-SAF's 310. The CoFeB layers 304 and 308 were also formed by PVD.

The functional metal layers 312 of Ta were formed by use of asacrificial metal layer (not shown) as discussed in relation to FIGS. 1aand 1b . The sacrificial metal layers consisted in the fabrication ofthe STT-MRAMs 300, 400 of 0.65 nm of Mg which were deposited, by meansof PVD, on top of the CoFeB layers 304. The CoFeB layers 304 had athickness of 1.5 nm.

During this PVD deposition of Ta on Mg, the light Mg atoms are removedby being sputtered by the impinging Ta atoms forming the functionalmetal layers 312. As a result a functional metal layer 312 comprising Tais formed on the respective CoFeB layers 304. The deposition of therespective functional metal layers 312 resulted in a layer thickness of1 nm.

After the respective PVD depositions, the substrates received a 30minute 300° C. crystallization vacuum anneal in presence of a 1 Teslamagnetic field in a TEL-MSL MRT5000 batch annealing system.

By annealing the respective STT-MRAMs 300, 400 at an elevatedtemperature of 250° C.-400° C., the layers 304 and 308 may at leastpartially crystallize into a [001] orientated layer, which may result inan increased TMR.

In the following the properties and performance of the STT-MRAMs 300,400 as well as similar device structures will be described.

FIG. 3a shows experimental data of measured TMR, indicated with opensymbols, for the STT-MRAMs 300, and 400, i.e. STT-MRAMs comprising Tafunctional layers 304 with a thickness of 1 nm which were formed byutilizing a Mg sacrificial metal layer with a thickness of 0.65 nm. TheTMR for the STT-MRAMs 300, 400 were measured to be about 20% higher thanthe TMR of a reference sample comprising a functional layer of Tadeposited without utilizing a sacrificial metal layer, illustrated bysolid symbols in FIG. 3a . FIG. 3a also shows that for both Co/Pt p-SAFstructures, square symbols, and ultrathin Co/Ni BP p-SAF structures,circle symbols, for pinning the reference layer 308 it is advantageousto use MTJs 302 comprising functional metal layers 312 which have beenformed using sacrificial metal layers.

FIG. 3b further illustrates that an increase in TMR may reach 35% whenthe thickness of the CoFeB layer 304, i.e. the free layer, is reduced toabout 1.3 nm. The decrease of the TMR may be attributed to the loss ofperpendicular magnetic anisotropy of the CoFeB layer 304. Since thedecrease occurs earlier for the functional metal layer 312 formed usinga Mg sacrificial metal layer it may, erroneously, be concluded that theearlier drop of the PMA is a result of the functional metal layer 312formed using a Mg sacrificial metal layer. However, this drop is notrelated to the weaker interfacial anisotropy resulting from thefunctional metal layer 312 formed using a Mg sacrificial metal layer.The earlier drop should, instead be attributed to a magnetic moment ofthe CoFeB layer 304 which is higher in the case of the functional metallayer 312 being formed using a Mg sacrificial metal layer indicatingthat a thinner dead moment is formed during Mg/Ta deposition.

Elastic recoil detection, ERD, measurements on films comprising CoFeBlayers 304 and functional metal layers 312 of Ta, where the functionalmetal layers 312 were formed using Ta sacrificial metal layers will bedescribed in the following.

FIG. 4a illustrates the measured Mg thickness extracted from ERDmeasurements for films comprising 1.5 nm thick CoFeB layers and Tafunctional metal layers with varying thicknesses, x. The respective Tafunctional metal layers were deposited on a Mg sacrificial metal layerhaving a thickness of 2 nm. FIG. 4a illustrates that the Mg is sputteredby about 2 nm during a 1 nm Ta deposition.

It may therefore be advantageous if the thickness of the sacrificialmetal layer is larger than the thickness of the functional metal layer.The thickness of the sacrificial metal layer may for example be largerthan but three times smaller than the thickness of the functional metallayer. By such an arrangement the sacrificial layer may be completelyremoved during the deposition of the functional metal layer.

The etching ratio when depositing Ta on Mg is about 2, i.e.approximately 2 nm of the Mg sacrificial metal layer is etched per 1 nmdeposited Ta functional metal layer.

FIG. 4b further illustrates measurements for films comprising 1.5 nmthick CoFeB layers and 1 nm thick Ta functional metal layers, where therespective functional Ta layers were deposited on Mg sacrificial metallayers of varying thicknesses, x. The ERD data shown in FIG. 4b showsthat no significant amount of Mg is left on the CoFeB layers, when usingsacrificial metal layers having thicknesses below 0.65 nm prior to thedepositing the 1 nm Ta functional metal layer.

The skilled person in the art realizes that the etching ratio may differfor different experimental conditions such as the processing equipmentused. In other embodiments the etch rate may for example be larger suchthat a thickness of the sacrificial metal layer smaller than 2 nm may beused where the sacrificial layer may be completely removed during thedeposition of the functional metal layer.

FIG. 4c illustrates experimental data for measurements obtained on CoFeBlayers onto which sacrificial metal layers of Mg, having differentthicknesses, where deposited. The thickness loss, i.e., the reduction inthickness, of the CoFeB layers is shown as function of the thickness ofthe deposited sacrificial metal layer after deposition of a functionalmetal layer of Ta with a thickness of 1 nm. The dotted line shows thereference value collected on a sample without Ta deposition. Thethickness loss data were extracted from Rutherford back scattering (RBS)measurements. The film thickness was calculated from the ERD and RBSmeasurement using the bulk densities of the material involved.

FIG. 4c shows that when no Mg is present, the CoFeB layer is removed byabout 0.17 nm when deposition 1 nm of Ta. It may further be concludedthat for thickness up to 0.65 nm the Mg sacrificial metal layer acts asa sacrificial protective layer in the sense that the layer is completelyremoved. For thicker Mg sacrificial metal layers thickness a reductionof the CoFeB layer may be reduced. However, when using a thicker Mglayer portion of the Mg layer may remain on the CoFeB layer which mayprovide undesirable properties.

In the above description the sacrificial metal layer was disclosed ascomprising Mg and the functional metal layer as comprising Ta. This isadvantageous as Mg atoms are light and the risk of damaging the topsurface of the layer may therefore be reduced. The Ta atoms are moreoverheavier than the Mg atoms such that the Ta atoms impinging on the Mg mayefficiently remove the Mg.

In other embodiments the sacrificial metal layer may comprise Al, Ca,Zn, or a combination thereof, which is advantageous in that thesacrificial metal layer may be removed by sputtering during thedeposition of the functional metal layer. Moreover, by this arrangementthe risk of damaging the top surface of the layer may further beminimized as Al, Ca and Zn are also light metal elements which may bedeposited with a reduced risk of damaging the top surface. Further, Al,Ca and Zn efficiently protect the top surface during the deposition ofthe functional metal layer.

To this end, the functional metal layer may comprise Co, Ti, W, Mo, Ru,Hf, or a combination thereof which is advantageous in that the methodmay be used to fabricate various types of devices comprising Co, Ti, W,Mo, Ru or Hf. The functional layer may, moreover, comprise thesematerials and Ta.

FIG. 5 illustrates experimental data for measurements obtained on CoFeBlayers onto which Ta layers have been deposited. The dashed curve inFIG. 5 illustrates the measured magnetoresistance as function ofannealing temperature for a layer structure where a sacrificial metallayer of Mg was used prior to the Ta deposition. The solid curveillustrates the measured magnetoresistance as function of annealingtemperature for a layer structure where Ta was directly deposited on theCoFeB.

From the experimental data it may be observed that when annealing thestacks up to 400° C., the Ta capped structures fabricated by using thesacrificial layer maintain higher perpendicular anisotropy relative tothe structures for which Ta was directly deposited on the CoFeB. The useof a sacrificial layer thereby improves the robustness during annealingand a better performance may be achieved.

In the following the method for protecting a top surface of a layer in asemiconductor structure will be discussed with reference to FIG. 6.

The method 500 comprises providing 502 a layer 104 on a substrate 102,the layer 104 having an initial thickness t₀ and an initial compositionC₀, and a top surface 108.

Next a sacrificial metal layer 106 on and in contact with the topsurface 108, is deposited 504 by means of physical vapor deposition,PVD. The sacrificial metal layer 106 deposited comprises a light metalelement.

A functional metal layer 110 is thereafter deposited 506 on and incontact with the sacrificial metal layer 106 by means of physical vapordeposition. During the deposition 506 of the functional metal layer 110the sacrificial metal layer 106 is removed by sputtering such that aninterface 112 is formed between the top surface 108 and the functionalmetal layer 106. The sacrificial metal layer 106 thereby protects thetop surface 108 during the deposition 506 of the functional metal layer110 such that, the layer 104 has a final thickness t_(f) matching orsubstantially matching the initial thickness t₀ and a final compositionC_(f) corresponding to the initial composition C₀.

The function and benefits of the method 500 are described above inrelation to the formation of the semiconductor structures 100, 200. Theabove mentioned features, when applicable, apply to the method 500 aswell. In order to avoid undue repetition, reference is made to theabove.

It should, however, be noted that the method 500 for protecting a topsurface 108 of a layer 104 in a semiconductor structure 100, 200 may beused from other semiconductor structures than the ones disclosed abovein relation to STM-MRAM devices.

The layer 104 may for example comprise a high-k dielectric material oran oxide. More specifically, the method 500 may advantageously be usedwhen the formation of an interface between a high-k dielectric or anoxide material and another material is crucial in the sense that thehigh-k dielectric or the oxide material need to be protected during theformation of other material such that the risk of damaging the high-kdielectric or the oxide material is reduced.

The person skilled in the art further realizes that the presentdisclosure by no means is limited to the preferred embodiments describedabove. On the contrary, many modifications and variations are possiblewithin the scope of the appended claims.

Additionally, variations to the disclosed embodiments can be understoodand effected by the skilled person in practicing the claimed invention,from a study of the drawings, the disclosure, and the appended claims.In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasured cannot be used to advantage.

What is claimed is:
 1. A method of fabricating a semiconductorstructure, the method comprising: providing a layer on a substrate, thelayer having an initial thickness and an initial composition; depositingby physical vapor deposition (PVD) a sacrificial metal layer on andcontacting the layer, the sacrificial metal layer comprising a metalelement having an atomic number of 36 or less; and depositing by PVD afunctional metal layer on and contacting the sacrificial metal layer,wherein during the deposition of the functional metal layer, at least aportion of the sacrificial metal layer is removed by sputtering suchthat an interface is formed in which the layer and the functional metallayer contact each other, and wherein the sacrificial metal layersubstantially protects the layer during the deposition of the functionalmetal layer, such that a final thickness of the layer substantiallymatches the initial thickness and a final composition of the layersubstantially matches the initial composition.
 2. The method accordingto claim 1, wherein the layer comprises Co, Fe, B or a combinationthereof.
 3. The method according to claim 2, wherein the sacrificialmetal layer comprises Mg, Al, Ca, Zn or a combination thereof.
 4. Themethod according to claim 1, wherein the functional metal layer is anamorphous metal layer.
 5. The method according to claim 1, wherein thefunctional metal layer comprises Ta.
 6. The method according to claim 1wherein an initial thickness of the sacrificial metal layer is less thanthan three times a final thickness of the functional metal layer.
 7. Themethod according to claim 6, wherein the initial thickness of thesacrificial metal layer is less than 2 nm.
 8. The method according toclaim 1, further comprising annealing the semiconductor structure at atemperature of 250° C.-400° C. to at least partially crystallizes thelayer into a substantially [001]-orientated layer.
 9. The methodaccording to claim 1, wherein the layer is a ferromagnetic layer. 10.The method according to claim 9, wherein the layer is a ferromagneticlayer having a perpendicular magnetic anisotropy.
 11. The methodaccording to claim 9, wherein the layer and the functional metal layerform part of a spin-transfer torque magnetoresistive random-accessmemory (STT-MRAM) stack.
 12. The method according to claim 11, furthercomprising forming a magnetic tunnel junction (MTJ), wherein the layerforms part of the MTJ.
 13. The method according to claim 12, furthercomprising forming an electrode adjacent to the functional metal layer.14. The method according to claim 1, wherein the layer is formed of ahigh-k dielectric material or an oxide.
 15. The method according claim1, wherein the functional metal layer comprises Co, Ti, W, Mo, Ru, Hf ora combination thereof.